It is known to provide projection objectives of different wide-opening fixed focal length adapted to be substituted for one another as required.
Since each objective lens for projection purposes requires a comparatively large number of individual lenses, the need for a number of such projection objectives having varying focal lengths results in a high cost for two or more such fixed focal length projection objectives as may be necessary for a particular purpose.
To avoid these problems, projection lens assemblies have been provided in a modular construction with a number of interchangeable lens groups which can be utilized with one another or as replacements for one another so that, by particular selection of the lens groups of the assembly, the respective focal length for the entire operative system in use at a given time can be chosen.
Such modular systems have, however, also been characterized by high cost and complications due at least in part to the relatively large number of lenses required in each lens group.